Before the Italian wolves started crossing back into France from Italy, they had been missing since the 1930s. However, France is a sparsely population country- its population is similar to the UK but in a country about 4 times bigger.
The wolf population has grown at a good rate for the last 25 years and now has reached between 500 and 600. The thing is that when wolves reach adolescence the usually leave their family pack and disperse looking for a mate, to set up a pack of their own. In good habitat, this dispersal may only be 20 miles. However in the current situation where humans have destroyed wolf populations around the world, that expansion often does not lead to the meeting that would allow a new pack to form.
In these instances the dispersing wolf can cover a huge distance. Dispersal of hundreds of miles is not unheard of.
As the wolf pack recovers in France, these dispersing wolves are likely to wander into areas that they have not been seen in for a long time.
In this instance, a wolf has been seen in northern France for the first time in a century. There is plenty of prey in this part of France, and while areas have a higher human population than much of their current range healthy populations are found in areas with far more humans. There is plenty of space, and more than enough wildlife for the wolf to eat. If the wolf population of France continues to grow (as it will unless humans stop it) they will eventually settle in northern France permanently. This is likely to have a positive impact on local biodiversity, and while some mitigation effort may be required there are many positive impacts that will likely occur, including a significant reduction in cars hitting deer.